1. Field of the Invention
The present invention relates to an optical head used in an optical information recording/reproducing apparatus which optically records and/or reproduces information with an information recording medium such as a magneto-optical disk. More particularly, the invention relates to an optical information recording/reproducing apparatus improved in its optical head portion effecting for recording and/or reproduction.
2. Related Background Art
Information record media employed in optical information recording/reproducing methods are used as external storage means for computers, because they have a large data record capacity in spite of their compactness. Among such media, magneto-optical information record media are more effective with respect of data rewritable properties. The mark position record method and the mark length record method (edge record method) are conventionally known as the method for information recording and/or reproduction with such storage means. It is understood that the latter is more advantageous than the former in that the data capacity is greater. In order to accurately reproduce information from a record medium recorded by the latter method, an edge position of an information bit must be accurately read by the optical head portion.
Normally, an optical information recording/reproducing apparatus used with a magneto-optical recording medium is operated such that an objective lens in an optical head focuses a beam from a semiconductor laser as a light source into a micro spot, data recording is carried out with this optical spot by the mark position record method, and such that information reproduction is performed by differential detection of a change in quantity of reflected light of the micro spot from the information bit. In such a conventional differential detection method, the reflected light is decomposed into two signal beams having different polarization components by a polarizing beam splitter having a polarization axis inclined at 45 degrees to the polarization direction of the incident linearly-polarized light. Two signals are detected from the two beams and processed to obtain a differential signal therebetween.
On the other hand, a problem will arise if a single micro spot having a light quantity distribution of conventionally known Gaussian distribution type is used in the mark length record method and if the differential detection is carried out with a light quantity change of reflected light thereof from an information bit. That is, the edge detectivity is low occur in detection of light quantity change for all reflected light beams, because the light quantity distribution of a light spot is of the Gaussian distribution having a certain spread. Means for solving the problem is already proposed as described for example in Japanese Laid-open Patent Application No. 3-268252, Japanese Laid-Open Patent Application No. 4-155640 or Japanese Laid-Open Patent Application No. 4-178941. In detail, the apparatus is so arranged such that a sectioned sensor detects a light quantity distribution in a far field or on a re-imaging plane thereby to obtain for example a spatial difference signal and then to obtain an edge detection signal with less jitters. On the other hand, there are various record methods, e.g., the magnetic field modulation and the optical modulation, proposed for magneto-optical recording of information.
The conventional examples as described above, however, still include the following unsolved problems. No consideration was made about an emission distribution of a light source in the above conventional examples. Then, the emission distribution and the oscillation mode are different between two excitation levels of a semiconductor laser corresponding to "0" and "1" in recording. Such differences will cause a change in light quantity distribution of a light spot on such a record medium as well as jitters being caused on a record edge due to the transient response during modulation. Also, jitters are seen in reproduction, which are specific to the edge detection and which are caused for example by a dispersion between laser elements, a change with time, unstable laser oscillation due to return light, and spatial unevenness which is a fluctuation of a diffraction pattern at respective apertures in a transmission optical system with wavelength change because of high-frequency superposition.